Phono II and III 3.18 time constant resistor value.

[bob orbell]

Hi all, its me again with some information, well a lot of information about the 3.18 time constant resistor value. This was first put in the FAQ for the WAD Phono II as 14454 ohms, I tried this mod and so did many others, I found the sound now far to bright as did others, so lets move along, I have been doing a shed load of research and calculations along with a old school friend who was fantastic at maths. Now, the time constant equation is t=RC which is 3.17988, this is not how it works as there are so many other factors and calculations to be taken into account, for example, preceding valve R out anode load resistor the 180k following valve grid leak resistor stray capacitance and more. So with the help of my chum and 5 sheets of A4 paper, a scientific calculator and many hours or work the figure we came up with is 2177.812 ohms, I fitted a 2K2 in Phono and have spent a lot of time listening, there is a welcome lift in HF and no sibilance at all. Comment's welcome please. Bob

[bob orbell]

And look what I just found after clearing out my very untidy workshop...... the original build instructions from Peter C for the Phono III s, take a look at the resistor at R22 just behind the black electrolytic capacitor next to C10, this was from Peter C. it is red red black brown - brown, 2K2, 1%, I rest my case. Bob

[A Stuart]

Originally Posted by bob orbell

with the help of my chum and 5 sheets of A4 paper, a scientific calculator and many hours or work

and from a different thread

Originally Posted by bob orbell

and why has no one posted a response to my finding of the 3.18 correct resistor value for Phono II III and IIIs ? are there no others who can do maths ? . Bob

- looks like you had already answered your own question.
Circuit theory makes sense (to me) when following an example in a book in front of me but making it work for the simplest of circuits is another matter. I come across 1970s school maths papers that I could walk through, compared with most classmates, and now need to think twice about what the question means never mind answer it.

I have previously had my suggestion that Thevenin theory be applied to a question (involving headphone impedance matching) ridiculed by a (former) forum member, yet he could probably have pieced together a soundly designed valve amp from a piece of wet string a light bulb and a old tin can.

It is satisfying to know when you have tied up a loose end like this rigorously out of your own (and chum's) efforts.

[Richard]

Hi Bob,
I've attached my circuits here for 5751 and ECC83 valves with cathode bypass caps fitted on the first valve stage. The valve type and bypass caps do affect the corner frequencies a little.
I used the 3S pcb which has the adjustable trimmer caps for the 3.18. I set these on SW and checked overall response using inverse RIAA filter and scope.
You'll see they end up very similar to the various versions of the published circuits, though there is a small (more accurate) change to corner frequencies (in theory by maths) due to returning to fitting the cathode bypass caps.
Without using the trimmers one could use the fixed cap values shown.

[vinylspinner]

Thanks Richard,
that is exactly how mine is setup using 5751 tubes. More than happy with it and have no plans to change anything, although I recently built a second Phono 3s for my second system but used low esr Panasonic caps as I’ve run out of the Sanyo Oscons.

[Richard]

Originally Posted by bob orbell

And look what I just found after clearing out my very untidy workshop...... the original build instructions from Peter C for the Phono III s, take a look at the resistor at R22 just behind the black electrolytic capacitor next to C10, this was from Peter C. it is red red black brown - brown, 2K2, 1%, I rest my case. Bob

Hi Bob,
Yes the pic in those Assembly Instructions shows R21/22 as 2k2 but the circuit shows R21/22 as 14k3.
The Operating Instructions shows a picture of R21/22 as 2k2 and in the circuit as 2k2.
My later paper copy of Matthew’s instructions shows both pictures and the circuit as 14k3.
I don’t know which R21/22 my (2) builds had. Mine were early builds supplied by Peter but I sold them maybe 10 years ago.

[bob orbell]

Thank Richard, I think that it was Dave Cawley that changed the value to 13K3 and from that time a lot of people who purchased from him mentioned issues with brightness and sibilance, now, all our (myself and my chum Brian's) calculations were done with JJ ECC83S, these valves were the original fitment to the WAD Phono II, and I still use them. With the 2K2 at R21-22 the scope trace is almost flat from 20 to 20K cycles, I have never tested with 14K3 but my ears tell me it is wrong, calculations show that the 75uS rise to 78.146uS when 14K3 is used, the 3.18uS rise to just over 5uS also there will be a small change to the 318 and 3180 pairing. The FAQ states 14454 ohms and this was calculated by someone (can't remember who) who simply used the time constant equation t = RC, this gives 3.17988uS, but that is not how it should be calculated, we need to know the thevenin resistance that the capacitor sees and that is more complicated but will give the correct value. Bob

[colin.hepburn]

Hi Bob/Richard/ all
Just out of curiosity, I had a look at my Phono 2 Schematic it refers to that resistor as R31/32 and has a 180K in there as opposed to the 14k3 in the Phono 3
I also have the JJ ECC83S fitted and the cathode cap fitted with the Oscon 25 volt 220uf and have no issues with brightness or sibilance.
Colin

[bob orbell]

No Colin, the Phono II does not include the 3.18Us time constant, there is no provision for it, but it is an upgrade in the FAQ, but the value is wrong, the 180K is present in all the Phono's, in the WD it is R19-20 followed by the 3.18 time constant resistor R21-22 which is what the debate is all about. Bob

[colin.hepburn]

Originally Posted by bob orbell

No Colin, the Phono II does not include the 3.18Us time constant, there is no provision for it, but it is an upgrade in the FAQ, but the value is wrong, the 180K is present in all the Phono's, in the WD it is R19-20 followed by the 3.18 time constant resistor R21-22 which is what the debate is all about. Bob

Hi BoB/All
yes sorry, you're right Bob my mistake to eager to try and help. I didn't check the phono 3 schematic as I should have done and mist the other resistors that's me out of this discussion I am now read only.
Colin